Quick Reply - "Close this thread" active area?

[spillage]

I'm having an issue with Mods accidentally closing threads when they quick reply to them.

The Close this thread checkbox selection area is active across the entire page, and is being accidently clicked in the process of trying to click the Post Quick Reply or Go Advanced buttons.
The Show your signature checkbox active area only extends a little way beyond its text.
Where do I look for, and how do I change the active area for the Close this thread checkbox?

TIA